Re: Maxphys on -current?

[mlelstv%serpens.de@localhost (Michael van Elst)]

gdt%lexort.com@localhost (Greg Troxel) writes:

>When you run dd with bs=64k and then bs=1m, how different are the
>results?  (I believe raw requests happen accordingly, vs MAXPHYS for fs
>etc. access.)

'raw requests' are split into MAXPHYS size chunks. While using bs=1m
reduces the syscall overhead somewhat, the major effect is that the
system will issue requests for all 16 chunks (1M / MAXPHYS) concurrently.
16 chunks is also the maximum, so between bs=1m and bs=2m the difference
is only the reduced syscall overhead.

The filesystem can do something similar, asynchronous writes are also
issued in parallel, for reading it may chose to read-ahead blocks to
optimize I/O requests, also for up to 16 chunks. In reality, large
contigous I/O rarely happens and the current UVM overhead (e.g. mapping
buffers) becomes more significant, the faster your drive is.

A larger MAXPHYS also reduces SATA command overhead, that's up to 10%
for SATA3 (6Gbps) that you might gain, assuming that you manage to
do large contigous I/O.

NVME is a different thing. While the hardware command overhead is
neglible, you can mitigate software overhead by using larger chunks
for I/O and the gain can be much higher, at least for raw I/O.